1. Field of the Invention
The present invention relates to a method of producing a substrate for an electron source by means of an offset printing technique, and to a method of producing an image-forming apparatus. More specifically, the present invention relates to a method of producing a larger-sized image-forming apparatus.
2. Related Background Art
In recent years, increasing intention has been given to an image-forming apparatus in the form of a thin flat panel which is expected to replace a cathode-ray tube (CRT) having disadvantages of great size and weight. Among various types of flat panel image-forming apparatus, liquid crystal display devices are extensively investigated. However, the liquid crystal display device still has a problem that the brightness of a displayed image is not high enough. Another remaining problem is that the view angle is limited to a narrow range. Emission type displays such as a plasma display device, a fluorescent display device, and a display device using an electron-emitting device are promising candidates for a display device that may replace the liquid crystal display device. These emission type display apparatus can offer a brighter image and a wider angle of view than liquid crystal display devices. On the other hand, there is a need for a larger-sized display device. To meet such a requirement, large-sized CRTs having a display area greater than 30 inches have been developed recently, and still greater CRTs are expected. However, the larger the display area of a CRT, the larger the space needed to install the CRT. This means that CRTs are not very suitable for providing a large display area. In contrast, flat panel display devices of the emission type with a rather small-sized body can offer a large display screen size, and thus they are now attracting the greatest intention. From this point of view, among various flat panel image-forming apparatus of the emission type, an image-forming apparatus using electron-emitting devices is very promising. In particular, the image-forming apparatus using a surface conduction electron-emitting device, proposed by M. I. Elinson et. al. (Radio. Eng. Electron. Phys., 10, 1290 (1965)) is attractive in that electrons can be emitted by a simple device.
In surface conduction electron-emitting devices, a thin film with a small size is formed on a substrate so that electron emission occurs when a current flows through the thin film in a direction parallel to the film surface. Various types of surface conduction electron-emitting devices are known. They include a device using a thin SnO.sub.2 film proposed by Elinson et. al., a device using a thin Au film (G. Dittmer, Thin Solid Films, 9, 317 (1972)), a device using a thin In.sub.2 O.sub.3 /SnO.sub.2 film (M. Hartwell and C. G. Fonstad, IEEE Trans. ED Conf., 519 (1975)), and a device using a thin carbon film (Araki et. al., Vaccuum, 26(1), 22 (1983)).
The device proposed by M. Hartwell et. al. is taken here as a representative example of a surface conduction electron-emitting device, and its structure is shown in FIG. 9. In FIG. 9, reference numeral 1001 denotes a substrate. Reference numeral 1004 denotes an electrically-conductive thin film which is formed of a metal oxide into an H pattern by means of sputtering. The electrically-conductive thin film 1004 is subjected to a process called energization forming, which will be described in greater detail later, so that an electron emission region 1005 is formed in the electrically-conductive thin film 1004. The portion of the electrically-conductive thin film 1004 between electrodes has a length L in the range from 0.5 mm to 1 mm and a width of 0.1 mm.
The inventors of the present invention have proposed a surface conduction electron-emitting device in which particles having the capability of emitting electrons are dispersed in a region between a pair of device electrodes, as disclosed in U.S. Pat. No. 5,066,883. This electron-emitting device has an advantage that electron emission positions can be controlled more precisely than the above-described other conventional surface conduction electron-emitting devices. FIGS. 3A and 3B illustrate a typical structure of the surface conduction electron-emitting device according to this technique disclosed in U.S. Pat. No. 5,066,883. This surface conduction electron-emitting device includes an insulating substrate 31, device electrodes 32 and 33 used to make electric connections, and an electrically-conductive thin film 34 containing electrically-conductive particles. An electron emission region 35 is formed in the conductive film 34. In this surface conduction electron-emitting device, the distance L between a pair of the device electrodes is preferably set to a value in the range from 0.01 .mu.m to 100 .mu.m, and the sheet resistance of the electron emission region 35 is preferably set to a value in the range from 1.times.10.sup.-3 .OMEGA./.quadrature. to 1.times.10.sup.-9 .OMEGA./.quadrature.. The device electrodes preferably have a thickness less than 200 nm so that the electrodes can have good electrical contact with the thin film 34 made of the conductive particles. When a great number of similar devices are arranged, it is important that there are small variations in the width and length of the portion of the thin film between the two electrodes so as to achieve small variations in the electron emission characteristics. FIGS. 4A to 4C illustrate the process of producing the electron-emitting device shown FIGS. 3A and 3B.
The inventors of the present invention have investigated a technique of achieving a greater-sized image-forming apparatus by disposing a great number of surface conduction electron-emitting devices on a substrate. There are various techniques to form an electron source substrate having electron-emitting devices and interconnections on the substrate. One of the techniques is to form all device electrodes and interconnections by means of photolithography. However, when the technique based on the photolithography is used to produce a large-sized image-forming apparatus, a large-scale exposure tool is required in the production. Furthermore, in this technique, a handling problem occurs and thus it is difficult to form a great number of devices having good characteristics with small variations on a substrate.
Another technique is to employ a printing technique such as a screen printing or offset printing technique to produce a circuit substrate. The printing technique is suitable for forming a pattern over a large area. Besides, this technique is inexpensive. An example of a technique of producing a circuit substrate by means of offset printing is disclosed in Japanese Patent Application Laid-Open No. 4-290295. In this technique disclosed in Japanese Patent Application Laid-Open No. 4-290295, the angles of plural electrodes for electrical connection to circuit components are varied so as to avoid an electrical contact failure due to the variation in the electrode-to-electrode pitch which arises from the expansion and contraction during a printing process. Furthermore, Japanese Patent Application Laid-Open No. 4-290295 discloses a technique of forming electrode patterns by means of offset printing.
However, if an electron source substrate is produced using a simple offset printing technique to form a large number of surface conduction electron-emitting devices on a substrate, great variations occur in the electron emission characteristics among the surface conduction electron-emitting devices disposed on the substrate. As a result, an image-forming apparatus obtained using this electron source substrate will have a poor image quality. This is mainly due to the variation in the shape of the device electrode across the substrate. In particular, there is a great variation in the shape between a central part and a peripheral region of the substrate.